-----BEGIN PGP SIGNED MESSAGE-----
=============================================================================
CERT(sm) Vendor-Initiated Bulletin VB-96.07
May 21, 1996
Topic: system stability compromise via mount_union program
Source: The FreeBSD Project, Inc.
To aid in the wide distribution of essential security information, the CERT
Coordination Center is forwarding the following information from the Free BSD
Project, Inc. The FreeBSD Project urges you to act on this information as soon
as possible. Their contact information is included in the forwarded text
below; please contact them if you have any questions or need further
information.
========================FORWARDED TEXT STARTS HERE============================
=============================================================================
FreeBSD-SA-96:10 Security Advisory
The FreeBSD Project, Inc.
Topic: system stability compromise via mount_union program
Category: core
Module: unionfs
Announced: 1996-05-17
Affects: FreeBSD 2.0, 2.0.5, 2.1, 2.1-stable, and 2.2-current
Corrected: (workaround) 2.1-stable and 2.2-current as of 1996-05-17
Source: 4.4BSD (lite)
FreeBSD only: no
Patches: ftp://freebsd.org/pub/CERT/patches/SA-96:10/
=============================================================================
I. Background
A bug was found in the union file system code which can allow
an unprivileged local user to compromise system stability.
This problem is present in all source code and binary
distributions of FreeBSD version 2.x released before 1996-05-18.
All FreeBSD users are encouraged to use the workaround provided
until the FreeBSD Project distributes a full solution.
II. Problem Description
The union filesystem code had problems with certain mount ordering
problems. By executing a certain sequence of mount_union commands,
an unprivileged local user may cause a system reload.
NOTE: This is a different problem than the one discussed in
FreeBSD SA-96:09. The workaround for this vulnerability is
similar to the one discussed in 96:09, but the proper solution
for the unauthorized access problem in 96:09 does not address
this vulnerability.
III. Impact
The problem could allow local users to compromise system stability.
This vulnerability can only be exploited by users with a valid
account on the local system.
IV. Solution(s)
The FreeBSD project is currently developing a solution to this
problem, however the proper solution will not be available until
a future FreeBSD release. We do not anticipate releasing patches
for previous versions of FreeBSD due to the extensive nature of this
fix. This security advisory will be updated as new information is
made available.
V. Workaround
This vulnerability can quickly and easily be limited by removing
the setuid permission bit from the mount_union program. This
workaround will work for all versions of FreeBSD affected by
this problem.
As root, execute the command:
% chmod u-s /sbin/mount_union
then verify that the setuid permissions of the files have been
removed. The permissions array should read "-r-xr-xr-x" as
shown here:
% ls -l /sbin/mount_union
-r-xr-xr-x 1 root bin 53248 Apr 26 04:40 /sbin/mount_union
In addition to changing the permissions on the executable files,
if you have the source code installed, we suggest patching the
sources so that mount_union will not be installed with the
setuid bit set:
*** /usr/src/sbin/mount_union/Makefile Sun Nov 20 14:47:52 1994
--- /usr/src/sbin/mount_union/Makefile Fri May 17 10:36:09 1996
***************
*** 8,14 ****
CFLAGS+= -I${.CURDIR}/../../sys -I${MOUNT}
.PATH: ${MOUNT}
- BINOWN= root
- BINMODE=4555
-
.include
--- 8,11 ----
=============================================================================
The FreeBSD Project, Inc.
Web Site: http://www.freebsd.com/
Confidential contacts: security-officer@freebsd.org
PGP Key: ftp://freebsd.org/pub/CERT/public_key.asc
Security notifications: security-notifications@freebsd.org
Security public discussion: security@freebsd.org
Notice: Any patches in this document may not apply cleanly due to
modifications caused by digital signature or mailer software.
Please reference the URL listed at the top of this document
for original copies of all patches if necessary.
=============================================================================
=========================FORWARDED TEXT ENDS HERE=============================
If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (FIRST).
We strongly urge you to encrypt any sensitive information you send by email.
The CERT Coordination Center can support a shared DES key and PGP. Contact the
CERT staff for more information.
Location of CERT PGP key
ftp://info.cert.org/pub/CERT_PGP.key
CERT Contact Information
- ------------------------
Email cert@cert.org
Phone +1 412-268-7090 (24-hour hotline)
CERT personnel answer 8:30-5:00 p.m. EST
(GMT-5)/EDT(GMT-4), and are on call for
emergencies during other hours.
Fax +1 412-268-6989
Postal address
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
USA
CERT publications, information about FIRST representatives, and other
security-related information are available from
http://www.cert.org/
ftp://info.cert.org/pub/
CERT advisories and bulletins are also posted on the USENET newsgroup
comp.security.announce
To be added to our mailing list for CERT advisories and bulletins, send your
email address to
cert-advisory-request@cert.org
CERT is a service mark of Carnegie Mellon University.
-----BEGIN PGP SIGNATURE-----
Version: 2.6.2
iQCVAwUBMaG4/nVP+x0t4w7BAQENJQP+OHXfr3rAdi6OaHY2b0VOI2ul8rZ5/dLV
8hHsO9xjU0z7kv+2hCDttEqYKfbfItrOPA+lTcx3VweKlFQZJI7hwqEneddzd4wB
b9FTjJNGzDBl+FgpD8vwHxxiTZ70AYg2vYeOYpvaFB982Z47NxXogre4DXv6r2Y9
DaauaJRNuyg=
=rSaQ
-----END PGP SIGNATURE-----